全文获取类型
收费全文 | 589篇 |
免费 | 10篇 |
国内免费 | 13篇 |
专业分类
化学 | 409篇 |
晶体学 | 2篇 |
力学 | 44篇 |
综合类 | 11篇 |
数学 | 47篇 |
物理学 | 99篇 |
出版年
2024年 | 2篇 |
2023年 | 5篇 |
2022年 | 19篇 |
2021年 | 23篇 |
2020年 | 18篇 |
2019年 | 17篇 |
2018年 | 19篇 |
2017年 | 11篇 |
2016年 | 24篇 |
2015年 | 19篇 |
2014年 | 19篇 |
2013年 | 49篇 |
2012年 | 52篇 |
2011年 | 48篇 |
2010年 | 24篇 |
2009年 | 24篇 |
2008年 | 26篇 |
2007年 | 35篇 |
2006年 | 23篇 |
2005年 | 23篇 |
2004年 | 8篇 |
2003年 | 21篇 |
2002年 | 28篇 |
2001年 | 6篇 |
1999年 | 8篇 |
1998年 | 6篇 |
1997年 | 5篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 2篇 |
1991年 | 5篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1973年 | 1篇 |
1970年 | 2篇 |
1968年 | 2篇 |
1961年 | 1篇 |
1960年 | 1篇 |
1959年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有612条查询结果,搜索用时 78 毫秒
541.
Anwar Saeed Niqab Khan Taza Gul Wiyada Kumam Wajdi Alghamdi Poom Kumam 《Molecules (Basel, Switzerland)》2021,26(21)
This research work aims to scrutinize the mathematical model for the hybrid nanofluid flow in a converging and diverging channel. Titanium dioxide and silver are considered as solid nanoparticles while blood is considered a base solvent. The couple-stress fluid model is essentially use to describe the blood flow. Therefore, the couple-stress term was used in the recent study with the existence of a magnetic field and a Darcy–Forchheiner porous medium. The heat absorption/omission and radiation terms were also included in the energy equation for the sustainability of drug delivery. An endeavor was made to link the recent study with the applications of drug delivery. It has already been revealed by the available literature that the combination of with any other metal can destroy cancer cells more effectively than separately. Both the walls are stretchable/shrinkable, whereas flow is caused by a source or sink with α as a converging/diverging parameter. Governing equations were altered into the system of non-linear coupled equations by using the similarity variables. The homotopy analysis method (HAM) was applied to obtain the preferred solution. The influences of the modeled parameters have been calculated and displayed. The confrontation of wall shear stress and hybrid nanofluid flow increased as the couple stress parameter rose, which indicates an improvement in the stability of the base fluid (blood). The percentage (%) increase in the heat transfer rate with the variation of nanoparticle volume fraction was also calculated numerically and discussed theoretically. 相似文献
542.
新疆北部干旱地区鼠类群落空间结构差异的比较研究 总被引:1,自引:0,他引:1
本文我们根据前人的研究并结合自己的调查,对新疆北部干旱地区鼠类群落空间结构差异进行了比较分析.结果表明,准噶尔盆地不同地区鼠类群落可划分为:高山草甸型、山地草原型、原生荒漠型、农田林地型和城市型等.一般高山草甸型以根田鼠为主;山地草原型以草原兔尾鼠、小林姬鼠、灰仓鼠为主;原生荒漠大沙鼠、子午沙鼠为主,梭梭荒漠型以大沙鼠,子午沙鼠为主,柽柳荒漠型以红尾沙鼠为主,跞石荒漠型以五趾跳鼠为主,荒漠草原型以五趾跳鼠、根田鼠为主,农田林地型以根田鼠、小家鼠、小林姬鼠、灰仓鼠为主;城市型以褐家鼠、小家鼠、灰仓鼠为主.同时发现,由于人类开垦经营,当地鼠类群落出现了由原始类型向人工环境的演变.褐家鼠是外来种,它已在不少地区占优势地位,成为当地主要的害鼠. 相似文献
543.
544.
Anwar Hossain Talukder 《Annals of the Institute of Statistical Mathematics》1968,20(1):441-455
The theory on regression estimate based on one auxiliary variable has been extended to that for more than one auxiliary variable. It has been found that the multivariate regression estimate (MRE) is not unbiased in general. The form of the approximate standard error of MRE is the same as that of simple regression estimate based on one auxiliary variable with the exception that the multiple correlation coefficient replaces the total correlation coefficient in the expression. It has also been found that the precision of MRE is non-decreasing, rather usually increasing as the number of auxiliary variables correlated with the dependent variable increases, assuming sample size to be large compared to the number of auxiliary variables. 相似文献
545.
The factors affecting the ultramicrodetermination of tryptophan and kynurenine were investigated by paper electrophoresis, paper chromatography and spectrophotometry. The electrolyte used in electrophoresis influences the mobility, the separation and the ultraviolet spectra of tryptophan and kynurenine. The pre-treatment of filter paper for chromatography did not influence the RF of tryptophan and kynurenine, but interfered with their quantitative determination. The effect of ultraviolet irradiation of d-, l-, and dl-tryptophan as well as d-, l-, and dl-kynurenine was studied. 相似文献
546.
S. S. Tahir Rauf Naseem Anwar ul Haq Khalid Saeed 《Accreditation and quality assurance》2005,10(7):362-368
This inter-laboratory comparison study was arranged for 28 laboratories from different public and private sector organizations
in Pakistan having wastewater testing capabilities aimed at improving the quality and comparability of test results. This
national inter-laboratory study was started in December 2003 and completed in July 2004. Laboratories were invited to analyze
the wastewater collected from printed circuit board (PCB) industry for lead and copper contamination. The samples fulfill
the criteria for homogeneity and stability as done by the reference laboratory. The results obtained from participating laboratories
were analyzed in terms of Hampel Test for outliers, while the performance evaluation of the participating laboratories was
done on the basis of Z-score. An assigned value derived from the participant's results was compared with a reference value provided by a reference
laboratory. Overall >50% of the participating laboratories have shown good performance in this PT-program 相似文献
547.
M.T. Rahman A.K.M.M. Hoque I. Siddique D.A.N. Chowdhury S.K. Nahar S.L. Saha 《Journal of organometallic chemistry》1980,188(3):293-300
Bromomagnesium diphenylcuprate and iodomagnesium dimethylcuprate have been prepared, and their thermal stability and some of their reactions investigated. In diethyl ether, 50% of the BrMgPh2Cu decomposes in 12h at room temperature and 50% of the IMgMe2Cu decomposes at 0°C in 14 but the cuprates are more stable in THF at or below 0°C. They react with acid chlorides to give 16–76% yields of the corresponding ketones. With bromine and iodine, BrMgPh2Cu gives bromobenzene and iodobenzene in 56 and 48% yield, respectively. Oxidation of BrMgPh2Cu with nitrobenzene or copper(II) chloride gives, respectively, 61 and 34% of biphenyl. Reaction of BrMgPh2Cu with acetic or benzoic anhydride yields 46–55% of the corresponding ketone and 34–37% of the corresponding carboxylic acids. 相似文献
548.
Syed Raza Shah Zarbad Shah Najeeb Ullah Javid Hussain Rashid Al-Harrasi Ajmal Khan Jeremy M. Rawson Ahmed Al-Harrasi Muhammad U. Anwar 《Acta Crystallographica. Section C, Structural Chemistry》2019,75(3):294-303
Reactions of 1,10‐phenanthroline (phen) and 2‐(3,4‐dichlorophenyl)acetic acid (dcaH) with Mn(CO3) (M = LiI, NaI and MgII; n = 1 and 2) in MeOH yield the mononuclear lithium complex aqua[2‐(3,4‐dichlorophenyl)acetato‐κO](1,10‐phenanthroline‐κ2N,N′)lithium(I), [Li(C8H5Cl2O2)(C12H8N2)(H2O)] or [Li(dca)(phen)(H2O)] ( 1 ), the dinuclear sodium complex di‐μ‐aqua‐bis{[2‐(3,4‐dichlorophenyl)acetato‐κO](1,10‐phenanthroline‐κ2N,N′)sodium(I)}, [Na2(C8H5Cl2O2)2(C12H8N2)2(H2O)2] or [Na2(dca)2(phen)2(H2O)2] ( 2 ), and the one‐dimensional chain magnesium complex catena‐poly[[[diaqua(1,10‐phenanthroline‐κ2N,N′)magnesium]‐μ‐2‐(3,4‐dichlorophenyl)acetato‐κ2O:O′] 2‐(3,4‐dichlorophenyl)acetate monohydrate], {[Mg(C8H5Cl2O2)(C12H8N2)(H2O)2](C8H5Cl2O2)·H2O}n or {[Mg(dca)(phen)(H2O)2](dca)·H2O}n ( 3 ). In these complexes, phen binds via an N,N′‐chelate pocket, while the deprotonated dca? ligands coordinate either in a monodentate (in 1 and 2 ) or bidentate (in 3 ) fashion. The remaining coordination sites around the metal ions are occupied by water molecules in all three complexes. Complex 1 crystallizes in the triclinic space group P with one molecule in the asymmetric unit. The Li+ ion adopts a four‐coordinated distorted seesaw geometry comprising an [N2O2] donor set. Complex 2 crystallizes in the triclinic space group P with half a molecule in the asymmetric unit, in which the Na+ ion adopts a five‐coordinated distorted spherical square‐pyramidal geometry, with an [N2O3] donor set. Complex 3 crystallizes in the orthorhombic space group P212121, with one Mg2+ ion, one phen ligand, two dca? ligands and three water molecules in the asymmetric unit. Both dcaH ligands are deprotonated, however, one dca? anion is not coordinated, whereas the second dca? anion coordinates in a bidentate fashion bridging two Mg2+ ions, resulting in a one‐dimensional chain structure for 3 . The Mg2+ ion adopts a distorted octahedral geometry, with an [N2O4] donor set. Complexes 1 – 3 were evaluated against urease and α‐glucosidase enzymes for their inhibition potential and were found to be inactive. 相似文献
549.
Hai‐Liang Zhu Xiu‐Ying Liu Xian‐Jiang Wang Feng Yang Anwar Usman Hoong‐Kun Fun 《无机化学与普通化学杂志》2003,629(11):1986-1990
Three polymeric silver(I) complexes with terephthalate anions as counterions or ligands, [Ag(pren)]2(tp)·2H2O ( 1 ), [Ag(en)][Ag(μ2‐tp)]·H2O ( 2 ), and [Ag2(μ4‐tp)(apy)2] ( 3 ) (where pren = 1, 2‐propylenediamine, tp =terephthalate dianion, en = ethylenediamine, and apy = 2‐aminopyridine) were synthesized and characterized by X‐ray single crystal analysis and infrared spectroscopy. 1 crystallizes in the monoclinic space group P211/c with a = 11.3221(5), b = 7.1522(3), c = 14.8128(5)Å, V = 1015.77(7)Å3, β = 122.132(2), and Z = 2. 2 crystallizes in the orthorhombic space group Pnma with a = 9.6144(6), b = 11.3465(7), c = 11.4810(7)Å, V = 1252.5(1)Å3, and Z = 4. 3 crystallizes in the monoclinic space group P21/n with a = 8.2003(5), b = 5.8869(4), c = 18.3769(11)Å, β = 92.593(1), V = 886.2(1)Å3, and Z = 4. Terephthalate dianions are not coordinated to the metal atoms in 1 , but act as a μ2‐bridging ligand in 2 and as a μ4‐bridging ligand in 3 . 相似文献
550.
Densities, ρ, viscosities, η, and refractive indices, nD, of glycine (Gly) (0.1 — 0.5 M) in aqueous 1,2‐ethanediol (1,2‐EtD), 1,2‐propanediol (1,2‐PrD), and 1,3‐butanediol (1,3‐BuD) (30% v/v) were measured at 298, 303, 308, and 313 K. Experimental values of ρ and η were used to calculate partial molar volumes, ?0v, partial molar volumes of transfer of Gly from water to aqueous diol solutions, ?0v(tr), Falkenhagen and Jones ‐Dole coefficients, A and B, respectively, free energies of activation of viscous flow, Δμ0*1 and Δμ0*2, per mole of solvent and solute, respectively, enthalpies, ΔH* and entropies, ΔS* of activation of viscous flow. Large positive values of ?0v, and an increasing value of Sv*, for all the three mixtures at each temperature suggest the presence of strong solute‐solvent interaction, and this interaction decreases as the size of alkyl moiety increases from 1,2‐EtD to 1,3‐BuD. Positive ?0v(tr) values tend to decrease with increasing the number of CH2 group, thereby indicating that the electrostriction effect in diols follows the sequence; 1,2‐EtD > 1,2‐PrD > 1,3‐BuD. Small A values, with large values of B, are indicative of weak solute‐solute and strong solute‐solvent interactions that operate in the present systems, and that the magnitudes of B are in the sequence: 1,2‐EtD > 1,2‐PrD > 1,3‐BuD and, thus, the sequence represents the strength of interaction between Gly and diol molecules. Moreover, positive SB/ST values suggest the structure‐breaking nature of Gly in diol + water mixtures. The observed values of Δμ0*2 fall in the sequence: 1,2‐EtD > 1,2‐PrD > 1,3‐BuD which, like ?0v and Sv*, reinforce that Gly‐diol interaction decreases with subsequent addition of CH2 group in diols. The trends in the variation of ΔH* and ΔS* with Gly concentration also reveal the presence of significant solute‐solvent interaction in all three systems. An almost linear increase in RD with an increasing amount of Gly reveals that Gly tends to increase the polarizability of the aqueous‐diol molecules under study. The variation of all these parameters with concentration of Gly and with temperature suggests the presence of strong solute‐solvent interaction, which decreases as the size of alkyl moiety in diols increases from 1,2‐EtD to 1,3‐BuD. 相似文献